My name is XXXXX XXXXX I'd be happy to answer your questions today. I'm sorry to hear about your son's situation.
If the wife is planning to take the children in two days, the first thing he can do is go to the courthouse and file an emergency motion for temporary custody, outlining the things you have said about the wife and explaining why it would be in the best interests of the children to stay with him instead of her. The judge has authority to say that, until the divorce is final and a full trial can be held, the children can stay with their father.
Since he is their father, and there isn't currently a custody order, he doesn't have any legal obligation to hand them over. However, it's better to go before the judge and have an order, because that will help him if she, for example, tries to pick the children up from school (he can alert the school) or shows up at his house with the police and try to take them.
If she is lying about her income, that is something that he will be able to bring up at mediation and/or the trial, when it happens. That will be a way of showing the judge that she shouldn't get spousal support
(and why the judge shouldn't believe her testimony).
As far as the divorce papers, he would file an Answer and Counterclaim, asking the judge to deny her custody, child support
, and spousal support, and asking the judge for a divorce. Here are the form and instructions:http://www.flcourts.org/gen_public/family/forms_rules/903c1.pdf
This site has additional forms and information, including financial affidavits and other documents that he will need to file.http://www.flcourts.org/gen_public/family/forms_rules/index.shtml